1. Field of the Invention
The present invention relates generally to semiconductor lasers (laser diodes). More particularly, the invention relates to an edge emitting semiconductor laser whose optical waveguide (which serves as an optical resonator) has parts with different widths, which suppresses the “beam steering” in the fundamental mode section of the waveguide to thereby raise the maximum fundamental-mode output, and a semiconductor laser module using the semiconductor laser.
2. Description of the Related Art
“Optical fiber amplifiers” play an important role as the relay or repeater in the wide-band optical communication system. Conventionally, an optical fiber doped with a rare-earth element has ever been used as the optical fiber amplifier. To excite the fiber of this type, a high-output semiconductor laser module is essential. A high-output, edge-emitting semiconductor laser (e.g., 0.98 μm band) used for thin must be optically coupled with a single-mode optical fiber with a high coupling efficiency. Therefore, with the semiconductor laser of this type, it is necessary to be operated in its stable fundamental mode. At the same time, it is necessary to have an emission spot matched to the light spot of a single-mode optical fiber with direct coupling or indirect coupling by way of a lens system, thereby realizing a desired optical coupling with a single-mode optical fiber at the emission edge of the laser.
With the high-output semiconductor laser of this type, generally, the width of the optical waveguide serving as the resonator is set at approximately 3.5 μm or less to allow the propagation of only the fundamental mode. The width of the waveguide is set constant along the propagation direction of light (i.e., along the longitudinal axis of the resonator) over the whole length. This is because the propagation mode (i.e., the lateral mode, which is parallel to the active layer) of light and its number allowed in an optical waveguide are determined by the width of the waveguide. This point will be explained below with reference to FIG. 1.
FIG. 1 shows the lateral mode change of propagating light dependent on the waveguide width W and the refractive index difference Δn in the optical waveguide of a conventional semiconductor laser of this type. In FIG. 1, the refractive index of the external part of the waveguide is 3.386 and constant over the whole length. The refractive index difference between the inside and outside of the waveguide is Δn. Thus, the refractive index of the waveguide is given as (3.386+Δn).
As seen from FIG. 1, the type and number of the allowable propagation modes (i.e., the lateral mode) of light are determined according to the width W and the refractive index difference Δn. In the example of FIG. 1, if the refractive index difference Δn is constant, only the fundamental mode (m=0) is propagated when the width W is sufficiently small. When the width W is increased slightly, the fundamental mode (m=0) and the first mode (m=1) are propagated. When the width w is further increased, the fundamental mode (m=0), the first mode (m=1), and the second mode (m=2) are propagated. In the same way, as the width W increases, the third mode (m=3), or the third mode and the fourth mode (m=4) will be additionally propagated. Thus, to propagate only the fundamental mode, the width W needs to be adjusted or selected according to the refractive index difference Δn.
With the above-described ordinary high-output semiconductor laser, the width W of the waveguide is constant along its axis and therefore, the following problems will occur.
Specifically, to obtain the stable lateral or horizontal mode, the waveguide needs to be narrowed. On the other hand, to realize a considerably wide spot size of the output light to cope with the high-output operation, it is effective to make the waveguide wide. As a result, if the width of the waveguide is constant, only one of these two requirements is realizable.
Here, the reason why to make the waveguide wide is effective to enlarge the spot size of the output light is explained below with reference to FIG. 2.
FIG. 2 shows the relationship between the waveguide width W and the FWHM (Full Width at Half Maximum) of the lateral or horizontal spot size of the conventional semiconductor laser used for FIG. 1, where Δn=3.5×10−3 (=0.0035).
As seen from FIG. 2, the lateral or horizontal spot size of the output light from the waveguide varies dependent on the width W of the waveguide. When the width W is larger than approximately, 1.5 μm, the FWHM of the horizontal spot size increases approximately proportional to the width W. Thus, to obtain a horizontally wide light spot of the output light, it is effective to increase the width w at the output end of the waveguide.
To satisfy the above-described two conflicting requirements (i.e., stabilization of the lateral mode and expansion of the spot size) simultaneously, various improved waveguides whose width is changed along its length have been developed and disclosed so far.
For example, with the semiconductor laser disclosed by the Japanese Non-Examined Patent Publication No. 9-307181 published in 1997, the optical waveguide serving as the resonator of a semiconductor laser is tapered. This laser comprises an active layer for emitting light, semiconductor cladding layers for confining light, a resonator structure for generating laser light, and a stripe-shaped, high refractive index region where the effective refractive index is higher than the other part, which are formed on a semiconductor substrate. The high refractive index region extends along the resonator. The width of the high refractive index region varies exponentially along the resonator. Thee width of the high refractive index region is 3.5 μm or less on one side of the resonator and 5 μm or greater on the other side thereof.
A current narrowing layer is selectively formed at each side of the tapered waveguide. The tapered geometry of the waveguide is realized by the current narrowing layer.
with the prior-art laser disclosed by the above-identified Publication No. 9-307181, the width of the waveguide at the front edge (i.e., the emission edge) where the density of light is high is set wide (at 5 μm or greater) to enlarge the light spot, thereby decreasing the density of light at the emission edge. Thus, degradation of the laser structure due to Catastrophic optical Damage (COD) or Catastrophic Optical Mirror Damage (COMD) is suppressed. On the other hand, the width of the waveguide at the rear edge where the density of light is comparatively low is set narrow (at 3.5 μm or less) to stabilize the lateral mode, thereby suppressing the generation of kink. “Kink” is a main factor that limits the high-output operation. Thus, kink-free output of 200 mW or higher is obtainable. As a result, mode loss due to mode conversion is prevented and the lateral mode is stabilized, which realizes high-level reliability in the high-output semiconductor laser.
In addition, with the prior-art laser disclosed by the Publication No. 9-307181, the width of the high refractive index region may be constant near the edge or edges.
The Japanese Non-Examined Patent Publication No. 8-340147 published in 1996 discloses a semiconductor laser having the same structure as that of the above-identified Publication No. 9-307181. The laser shown in the Publication No. 9-307181 seems to utilize the laser shown by the Publication No. 8-340147.
The Japanese Non-Examined Patent Publication No. 8-23133 published in 1996 discloses a semiconductor laser whose waveguide serving as the resonator has a varying width along its length. This laser has a ridge-type waveguide structure for controlling the lateral mode of propagating light. The active layer is narrowed by selectively removing the active layer or by selectively forming recesses at each side of the ridge-type waveguide, thereby suppressing the radiation mode. Thus, there are the advantages that the laser operates stably at the lateral fundamental mode even when the output level of light is as high as the watt class, the fabrication yield is high, and the reproducibility of characteristics is good.
The Japanese Non-Examined Patent Publication No. 9-289354 published in 1997 discloses a high output, low threshold current semiconductor laser having a large horizontal spot diameter. This laser comprises a semiconductor layered structure formed on the semiconductor substrate. The layered structure includes the active layer which is stripe-shaped along the resonator. The width of the active layer is W1 at the front edge and W2 at the rear edge, where W1>W2. The width varies continuously from W2 to W1 along the resonator. Thus, laser light with a large horizontal spot diameter is generated.
With the prior-art laser disclosed by the above-identified Publication No. 9-289354, the width W1 at the front edge (i.e., the emission edge) is set to be approximately equal to the spot diameter of the light that propagates through an optical fiber to be coupled with the laser. The width W2 at the rear edge is set in such a way that laser oscillation occurs at the single lateral mode.
The Japanese Non-Examined Patent Publication No. 5-267772 published in 1993 discloses a ultra high output, lateral mode-controlled semiconductor laser. This laser, which is designed for a light source for SHG (Secondary Harmonic Generation), has a broad area structure at its edge. A narrow stripe-shaped part is formed in the cavity as the gain-guided waveguide structure.
With the prior-art laser disclosed by the above-identified Publication No. 5-267772, the broad area structure is provided at the emission edge and therefore, the spot size of light at the same edge is enlarged. This means that degradation of the laser at the emission edge does not occur and high output operation is possible. Since the narrow stripe-shaped part is formed in the cavity, higher modes can be removed due to the mode filtering function. Since the narrow stripe-shaped part has the gain-guided waveguide structure the propagating light will expand efficiently along the waveguide at the interface of the broad area region and the narrow stripe region. Thus, the fundamental mode is efficiently selected.
With the above-described prior-art lasers disclosed by the Publication Nos. 9-307181, 8-340147, and 9-289354, a stable lateral mode is obtainable and at the same time, a considerably wide light spot is possible while taking the coupling with an optical fiber into consideration.
With the above-described prior-art laser disclosed by the Publication No. 8-23133, the coupling with an optical fiber is not referred. However, it seems that a stable lateral mode is obtainable and at the same time, a considerably wide light spot is possible while taking the coupling with an optical fiber into consideration.
With the above-described prior-art laser disclosed by the Publication No. 5-267772, a stable lateral mode is obtainable. However, it seems that the laser is difficult to be coupled with an optical fiber, in particular, a single-mode optical fiber. This is because the stripe-shaped part is set wide at the emission edge to prevent the degradation at the same edge.
According to the inventor's examination, the above-described prior-art semiconductor lasers have the following problems.
With all the above-described prior-art lasers, a specific current is supplied in such a way as to be perpendicular to the optical 1 waveguide (i.e., the resonator) on operation. Therefore, even if the width of the fundamental mode section (i.e., the part with a relatively narrow width) of the waveguide is designed in such a way that only the fundamental mode is propagated on oscillation, there will be a problem that the fundamental mode is unable to be kept when the injection current density exceeds a certain level (e.g., 2×104A/cm2). The reason of this problem is as follows.
The gain distribution and the refractive index distribution of an optical waveguide are not always constant but are likely to fluctuate with the increasing injection current density. If the injection current density is very high, the gain distribution and the refractive index distribution will deviate distinctly from their predetermined ones. This deviation corresponds to generation of a new gain distribution and a new refractive index distribution. These new distributions thus generated will allow the propagation of higher modes than the fundamental mode. This point is explained, for example, in the book entitled “semiconductor laser (basis and application)”, written by Ryoichi Ito and Michiharu Nakamura, published by Baihukan, 1989, pp. 97.
Accordingly, as disclosed in the paper, IEEE Photonics Technology Letters, Vol. 6, No. 12, December 1994, pp. 1409-1411, there arises a possibility that the propagating light, which has been going straight, is bent. This means that the direction of the output light beam may be changed. This phenomenon is termed the “beam steering”.
Because of the above-described mechanism, even if the fundamental mode section (i.e., the narrow part) of the optical waveguide is designed for allowing the propagation of only the desired fundamental mode, if the injection current density exceeds a certain level (e.g., 2×104A/cm2), a problem that the direction of the output light beam may be changed will occur. This problem decreases the coupling efficiency of the laser with an optical system such as an optical fiber, thereby lowering the utilization efficiency of the output light.
Additionally, the maximum output of a high-output semiconductor laser is limited by “thermal saturation”. “Thermal saturation” is caused by the fact that Joule heat generated in a semiconductor laser by the injection current induces saturation of the gain of the laser.